The invention provides α-amino esters of a hydroxypropylthiazolidine carboxamide derivative, (2S)-3-([1,1′-biphenyl]-4-ylsulfonyl)-N-[(1S)-3-hydroxy-1-phenylpropyl]-1,3-thiazolidine-2-carboxamide, as well as salts and crystal polymorphs thereof, that can be used to inhibit prostaglandin F receptor. The invention further encompasses methods of treating disorders such as pre-term labor at the early gestational stage by the administration of these substances to a patient in need of treatment.